1. Field of the Invention
The present invention relates to an image processing apparatus which can perform expansion/reduction processing of a document image including both character and photographic images, while ensuring both a good resolution of a character portion and a good gradation of a photographic region.
2. Description of the Related Art
In a document image processing apparatus which can handle image information as well as code information, binary quantization of document information read by a read means, such as a scanner, is generally performed as follows. With respect to an image information having contrast, such as characters and line drawings, simple binary quantization is performed by using a fixed threshold value. With respect to image information having a gradation, such as photographs, binary quantization is performed by a pseudo-gradation representing means, such as the dither method. Such a means is employed for the following reason. If simple binary quantization is performed with respect to read image information by using a fixed threshold value, no deterioration in image quality occurs in character/line drawing image regions because resolution is preserved, but deterioration in image quality occurs in photographic image regions because the gradation is not preserved.
In contrast to this, if gradation processing such as the ordered dither method is performed with respect to read image information, no deterioration in image quality occurs in photographic image region because the gradation property is preserved, but a deterioration in image quality occurs in character/line drawing image regions because the resolution property deteriorates. That is, if a single binary quantization method is performed with respect to the read image information, high image quality cannot be ensured in regions with different features at the same time. For this reason, when a document on which character and photographic images are formed together is to be processed, a target image is separated into a character region and a photographic region, and the character and photographic regions are processed by selectively or adaptively switching between the simple binary quantization mode and the dither processing mode.
When image information is to be output upon expansion or reduction, multi-value image information input through a scanner or the like is expanded/reduced in advance, and the above-described binary quantization is performed. This is because the expansion/reduction processing after binary quantization is inferior to that of the multi-value image information in terms of the precision of expansion/reduction processing. As such expansion/reduction processing, the linear interpolation method, the projection method, and the like are known.
In these methods, however, since the image information of neighboring picture elements is interpolated, changes in density of an image cannot be faithfully reproduced. In the above-described image region separation method, changes in density of an image are used. Therefore, image region separation cannot be performed with respect to an image which has undergone the conventional expansion/reduction processing. That is, a conventional apparatus cannot perform expansion/reduction processing of a document on which character and photographic images are formed together while ensuring both good resolution of a character portion and good gradation of a photographic region.